In recent years, there has been a great deal of interest in the application of very low power analog/digital circuitry in battery powered biomedical systems. Much of this interest has centered around implantable cardiac pacemakers. These devices are used to treat cardiac arrhythmias, such as bradycardia (slow heart rate) or tachycardia (rapid heart rate), by assisting the heart's natural pacemaking function with relatively low voltage (5-10 V) pulses.
This type of treatment is, however, totally ineffective against fibrillation, which is characterized by very rapid, uncoordinated electrical activity in which the heart essentially stops pumping blood. Death quickly results unless emergency treatment, generally in the form of a defibrillating high voltage shock, is administered. It is estimated that each year in the U.S. about 300,000 people suffer sudden death from this condition.
As the technology for defibrillators improves, it becomes possible to more completely combine both pacing and defibrillation capabilities in a single implantable device. This results in significant performance improvements.
It is well recognized that biphasic defibrillation is more effective than monophasic defibrillation. Winkle et al., "Improved low energy defibrillation efficacy in man with the use of a biphasic truncated exponential waveform", American Heart Journal, January 1989, pp. 122-127.
U.S. Pat. No. 4,800,883 issued Jan. 31, 1989, to William L. Winstrom, discloses an apparatus for generating a multiphasic defibrillation waveform. The Winstrom apparatus is suitable for use in an implantable defibrillation system for automatically generating a multiphasic defibrillation pulse waveform in response to sensed fibrillation. A controller senses cardiac fibrillation and generates a control signal that causes a charging circuit to charge two series charge-storing capacitors to selected voltage levels in sequentially alternating charge generation and charge coupling cycles. A voltage level detector senses the stored voltage level, disables the charging circuit when the sensed voltage reaches a predetermined level and informs the controller that the capacitors are fully charged. The controller then communicates control signals indicative of pulse magnitude, duration and polarity to a multiphasic pulse generator that includes a number of high-power switches and corresponding switch drivers interposed in-circuit between the heart and the terminals of the charge storing capacitors. The drivers control the conduction states of the switches according to the control signals to establish selected circuit paths between the capacitor terminals and the heart, thereby delivering to the heart a multiphasic waveform having pulses with selected parameters of magnitude, duration and polarity.